1. Field of the Invention
This invention generally relates to a sealing arrangement for joining at least one ceramic component to another component. More particularly, this invention relates to a multi-layer seal arrangement for establishing high temperature seals where at least one ceramic component is joined to another component.
2. Description of the Related Art
There are various structures that are made from ceramic components, which are joined together. In most circumstances, a reliable seal must be established between the ceramic components for the arrangement to work as intended. Conventional techniques for joining ceramic-to-ceramic or ceramic-to-metal components involve such practices as active-metal brazing, which consists of a single braze alloy comprised of a base metal (such as silver or gold) and a chemically reactive metal (such as titanium, zirconium, or hafnium). The base metal provides the bulk of the braze structure, which is usually ductile, while the active metal promotes interfacial wetting and bonding between the components during the brazing process.
A significant challenge arises in the case of a ceramic oxygen generator, where a plurality of individual cell components is joined together. A cell consists of a thin solid ceramic electrolyte that is coated on each side with a gas permeable (porous) electrode material such as platinum. The ceramic electrolyte is also sandwiched between and bonded to two interconnecting plates. These plates, or interconnects, make electrical contact with the electrodes, and also contain channels that direct the flow of the different gases over the electrodes. The brazing material is in the shape of a gasket, which forms a non-contacting boundary around the electrodes, and is used to join the electrolyte surfaces to the interconnect plates. Each cell, and the stacked cells, must be reliably sealed and bonded together in order for them to withstand the temperatures associated with oxygen generation and with other operating conditions. Another requirement of the brazing gasket material, is that it must compress sufficiently in the presence of a liquid phase during the brazing process so that the interconnect plates will make good electrical contact with each electrode.
In conventional brazing processes, which typically use a single active-metal brazing alloy, detrimental interfacial reactions can occur and be very difficult to control, depending on the exact materials to be joined. For example, extensive reactions have been observed in zirconia/active-metal braze interfaces. During the brazing process, the active metal migrates to and chemically reacts with the zirconia surface. This reaction causes pitting and stress concentrations in the zirconia electrolyte and causes premature failure. Thus for ceramic oxygen generators and other electrochemical devices (such as solid oxide fuel cells), the challenge is in establishing an appropriate seal without compromising the material at the sealed interfaces.
It is therefore necessary to control the extent of the interfacial reactions so that they provide sufficient wetting and bonding, but at the same time do not degrade the properties of the ceramic components being joined. This invention addresses that need, while avoiding the shortcomings and drawbacks of the prior art.
Also addressed is one prior art approach where a ceramic stopper has been incorporated into the braze arrangement in order to control the flow of the braze alloy during the brazing operation. The stopper allows for greater latitude in the brazing temperature by confining the flow of the alloy to the seal area and thereby preventing electrical shorting to the electrodes.